1. Field of the Invention
The subject invention relates to RF (radio frequency) systems and more particularly to an RF architecture for a multi-band cellular telephone.
2. Description of Related Art
Mobile phones have recently gained widespread use throughout the world. Mobile phones communicate with a base station serving a predetermined area or cell of a cellular network system, such as GSM. Each base station has a limited bandwidth within which to operate, depending upon the particular transmission technique utilized by the base station. This limited bandwidth is separated into a plurality of channels, which are frequency-spaced evenly from one another, and these plurality of channels are used by the mobile phones within that base station""s transmission area. As a result, each base station can only handle a limited number of phones. The number of possible phones is equal to the number of channels and time-slots on those channels available at the base station. Therefore, the frequency spacing between channels is minimized in order to maximize the number of channels supported by the operating bandwidth of the base stations.
The capacity of base stations in highly populated areas can become saturated during time periods of high use. Mobile phones currently operate as single band phones, where the transmitted signal frequency is within the bandwidth of a base station operating on the same transmission method as the mobile phone. Thus, there is a need for mobile phones to operate with multiple band transmissions to increase system capacity, so that the system could select between multiple transmission frequency bands depending upon which bandwidth is less saturated and could provide a better signal quality.
Typically, in a conventional single band cellular phone, data to be transmitted by the telephone handset is fed to a transmitter including a differential encoder, where an in-phase component (I) and a quadrature component (Q) of the signal to be transmitted are created. The I and Q components are then passed through digital filters which give the modulation a particular shape. The resultant I and Q filtered signals are then modulated at a radio frequency for transmission and combined as a phase modulated signal. The phase modulated signal is then amplified to bring the signal to a desired power level for transmission. Digital modulators, such as a Gaussian minimum shift keying (GMSK) modulator, are typically used in digital wireless phones.
Most mobile phones are designed to be lightweight and portable, so that they may be easily carried on the person using the mobile phone, such as in their pocket or purse. It is therefore critical to design a mobile phone to be as small as possible, thus requiring the number of components to be minimized. With respect to design of a multi-band mobile phone, these considerations present serious design problems, for example, because the use of entirely separate transmitter and receiver circuits for the respective bands would result in a prohibitively large and complex phone. Additional problems confront implementation of a multi-band phone arising from the potential for generation of unwanted frequency components.
It is therefore an object of the present invention to improve mobile phone systems;
It is another object of the invention to provide a multi-band RF transmitter/receiver architecture;
It is a further object to improve RF architecture of mobile phone systems;
It is another object of the invention to provide a multi-band RF cellular phone architecture;
It is another object of the invention to provide such an RF multi-band architecture employing the DCS 1800, GSM, and PCS 1900 frequency bands;
It is another object of the invention to provide a multi-band RF cellular phone architecture which adds as little circuit complexity as possible as compared to a single band design;
It is another object of the invention to provide a multi-band RF cellular phone architecture which minimizes spurious (unwanted) frequency problems.
According to a first aspect of the invention, a multi-band RF architecture is provided including a modulator device for modulating an intermediate frequency (IF). The frequency of the modulated IF signal is changed such that a respective IF is used for each of the GSM, DCS 1800, and PCS 1900 bands. An intermediate frequency (IF) filter with a pass band that covers the GSM IF, the DCS 1800 IF, and the PCS 1900 IF is connected to the output of the modulator device and outputs to a transmit phase lock loop. The transmit phase lock loop translates the IF signal from the IF filter to either a DCS 1800 band radio frequency signal, a GSM band radio frequency signal, or a PCS 1900 band radio frequency signal, depending on the frequency of a local oscillator (LO) and whether the GSM transmit VCO, the DSC transmit VCO, or the PCS transmit VCO is active.
According to a second aspect of the invention, the RF architecture employs a down converter for receiving a GSM band signal, a DCS 1800 band signal, or a PCS 1900 band signal, and for supplying a down converted output signal at an output thereof, the down converted output signal being selected by high side injection for the GSM band and by low side injection for the DCS 1800 and PCS 1900 bands.
According to yet another feature of the invention, the down converter outputs to an intermediate frequency (IF) receiver filter designed to pass either the down converted GSM signal, the down converted DCS signal, or the down converted PCS signal. A particularly important inventive aspect of the design is that the receiver IF filter is centered at 400 Megahertz (MHz), which contributes to numerous advantages and simplification in the circuitry.
According to yet another aspect of the invention, a single phase lock loop circuit is used to supply, on a single output, the LO signal for down converting either the GSM receive band, the DCS 1800 receive band, or the PCS 1900 receive band, as well as to the transmit phase lock loop. In this manner, a common phase lock loop is used for down conversion in both the transmit and receive paths of the circuitry.
Those skilled in the art will appreciate the considerable number of advantages arising from the architecture of the preferred embodiment. It employs a common IF (Intermediate Frequency) filter for all RX bands, and a single transmit PLL (Phase Locked Loop) for up-converting the phase-modulated IF for all TX bands. The architecture further employs a common IF VCO for both the receive and transmit path. Finally, a common PLL is used for generating the LO (Local Oscillator) signal for the first down-conversion in the receive path and the down-conversion in the PLL of the transmit path. Thus, the frequency plan and other aspects of the RF architecture permits the same IF filters, mixers, VCOs and PLLS to be used regardless of the active band.
The disclosed architecture achieves the goal of adding as little circuitry as possible as compared to a single band design while minimizing effects of unwanted (spurious) frequency components. Thus, the preferred embodiment exhibits superior operational capabilities combined with greatly reduced circuit complexity.